The present invention relates to a thin pressure sensor for measuring pressure such as fluid pressure and contact pressure of objects. The invention also relates to a biological information measuring device and a biological information measuring method, using such a thin pressure sensor.
Mainstream pressure sensors are adapted to measure deformation of a diaphragm under pressure using a strain gauge or a laser. The applied pressure on the diaphragm is detected from a correlation between the degree of deformation and the pressure. However, in this type of pressure sensor, the diaphragm requires a supporting mechanism and the structure is complex. Accordingly, the thickness of the pressure sensor can only be reduced to several millimeters and it was impossible to achieve a thickness of 0.6 mm or less.
Another type of pressure sensor uses a pressure sensitive material to measure an electric resistance shift that is generated when pressure is applied. A value of the applied pressure is computed from a correlation between the electric resistance and the pressure. This type of pressure sensor does not tolerate a thickness of 100 xcexcm or less because such a thickness impairs durability and mechanical strength.
One application of the pressure measurement is the living body. For example, information of the living body is obtained by measuring changes in pulse, respiration, or body movement of the subject. This is often carried out by measuring changes in pressure of the air-mattress that is placed under the subject laying on his or her back, or by measuring oscillations of wires that are suspended under the pillow. One drawback of such measuring methods, however, is that the sensor can be placed only at particular locations and the methods are only applicable to the living body at rest. In view of such a drawback, there have been attempts to obtain biological information using an ordinary pressure sensor. However, the ordinary pressure sensor is not suitable because it has a hard sensor head, which gives discomfort to the subject.
The present invention was made in view of the foregoing problems and it is an object of the present invention to provide a novel thin pressure sensor that has a simple and thin structure with sufficient durability and sufficient mechanical strength. Another object of the present invention is to provide a novel biological information measuring device that uses the novel thin pressure sensor of the present invention, and which can solve the drawbacks of conventional biological information measuring devices.
After extensive research on thin pressure sensors and applicability of such pressure sensors to a biological information measuring device, the inventors of the present invention have found that the foregoing objects can be achieved by placing a single internal electrode between a pair of thin film external electrodes that have piezoelectric layers on the inner sides, and by completely shielding the internal electrode from outside between the pair of external electrodes.
Namely, the present invention provides a thin pressure sensor that includes: a pair of external electrodes (1, 1xe2x80x2), which are respectively made of conductive thin films (3, 3xe2x80x2) that are respectively provided with piezoelectric layers (2, 2xe2x80x2) on inner sides; and a single internal electrode (4), made of a conductive thin film, which is sealed between the pair of external electrodes, one of the pair of external electrodes having a conducting window (5) that conducts to the internal electrode (4). The present invention also provides a biological information measuring device that measures a pressure change of a living body using such a thin pressure sensor. The present invention also provides a biological information measuring method that measures a pressure change of a living body using such a thin pressure sensor.
The present invention thus realizes a thin and highly sensitive pressure sensor. A plurality of such a thin pressure sensor may be provided in the form of a matrix over a flat or curved surface. In this way, a two-dimensional pressure distribution can easily be measured. Further, the thin pressure sensor can be used in direct contact with a living body such as the human body. Further, the thin pressure sensor easily fits non-flat surface shapes of the living body. As a result, biological information, such as pulse, respiration, and body movement, can be measured sensitively in non-restrictive and non-invasive manners.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.